The present invention mainly relates to a portable small-sized wireless device and, particularly to a technique for improving the receiving sensitivity of a whip antenna.
Recently, the demand has been increased abruptly as to a mobile wireless device such as a-cellular phone, a PHS, a PCS or the like.
A whip antenna capable of being retracted within the body of a telephone has been known as an antenna for a conventional portable telephone.
FIG. 7 is a front view showing the configuration of a conventional small-sized wireless device. As shown in FIG. 7, a whip antenna 71 is used as an antenna serving both as a receiving and transmitting antenna retracted within a small-sized wireless device body 72, and is usually provided at the upper portion of the rear face of the small-sized wireless device body 72 so as to obtain good performance in a state where a person uses the apparatus using a receiver 73. As a result, it is possible to perform high-quality and stable mobile communication.
According to the increase of the market for the mobile communication such as a cellular phone, a PHS, a PCS or the like, smaller-size goods with better performance have been demanded.
Recently, the main body of the telephone has become thinner due to the miniaturization thereof. Accordingly, the whip antenna 71 has become closer to the receiver 73 as shown by the using state of the telephone in FIG. 8. Accordingly, the distance between the whip antenna 71 and the head portion 74 of a human body tends to become shorter at the time when a person holds the telephone and abuts the telephone against his ear in use.
However, it has been known that when the whip antenna 71 approaches the head portion 74 of a human body, the head portion 74 which is dielectric substance of high permittivity absorbs radio wave, so that both the antenna performance and the communication quality degrade.
FIG. 9 is a graph showing the relation between the distance D [mm] between the position of the whip antenna 71 and the head portion 74 of a human body and the radiation gain of the antenna. It is understood from the figure that the shorter the distance D [mm] (abscissa) is, the more remarkably the radiation gain of the antenna (ordinate) degrades. In this regard, the degree of the degradation due to the head portion 74 of a human body is larger in the retracted state of the whip antenna 71 within the main body (symbol X) as compared with the extended state of the antenna (symbol of black rectangular).
Accordingly, in recent years, although the small-sized wireless device body 72 becomes thinner, the whip antenna 71 is disposed at a portion as closer as possible to the rear surface of the main body so that the distance between the whip antenna and the head portion 74 of a human body becomes longer.
However, if the small-sized wireless device is designed in the above manner, the antenna is very close to the surface of a desk or the like on which the portable small-sized wireless device is placed when it is laid on its rear face on the desk so as to wait to receive a call. Accordingly, the receiving sensitivity of the antenna will be degraded remarkably when the portable apparatus is placed on a metal desk such as a steel desk.
In particular, in the case of a thin rod-shaped cellular phone, the cellular phone is likely laid on a desk to wait to receive a call while its display surface 75 of an LCD (liquid crystal display) is turned upward as shown in FIG. 10. In this case, when the cellular phone is placed on a metal desk such as a steel desk, a metal plate 76 of the desk is close to the whip antenna 71. Consequently, a current I' [mA] whose phase is in opposite to a current I [mA] on the whip antenna 71 flows on the surface of the metal plate due to the mutual coupling of the metal plate and the whip antenna. As a result, there arises a problem that, the radiation direction characteristic of the whip antenna 71 changes and the impedance characteristic thereof changes greatly, whereby the matching state of the antenna is broken and so the radiation gain is degraded.